Fix Analyze issues in Firestore

When running Product -> Analyze in Xcode

<img width="649" alt="screen shot 2018-08-24 at 6 54 16 am" src="https://user-images.githubusercontent.com/73870/44588442-92703800-a76a-11e8-9184-85f0d9f5fee5.png">

I also filed https://github.com/grpc/grpc/issues/16451